gg5, prog, daij

Doesn't appear to be adverbial.

@@ -5,0 +6,2 @@
+<ke_pri>news2</ke_pri>
+<ke_pri>nf38</ke_pri>
@@ -9,2 +10,0 @@
-<ke_pri>news2</ke_pri>
-<ke_pri>nf38</ke_pri>
@@ -35,3 +35,3 @@
-<pos>&adv;</pos>
-<gloss>in passing another (esp. colliding with oncoming traffic, bumping into a person)</gloss>
-<gloss>the moment two persons or objects meet</gloss>
+<gloss>the moment of meeting (of two people, vehicles, etc.)</gloss>
+<gloss>the moment of running into each other</gloss>
+<gloss>the moment of crossing paths</gloss>

GG5 has both, in this order.

Meikyo and smk lead with the 会 version, which also gets the most counts below

〈出／で〉〈会（い）／合（い）／あい〉〈頭／がしら／ガシラ〉

Google N-gram Corpus Counts
╭─ーーーーーー─┬────────┬───────╮
│ 出会い頭　　 │ 42,253 │ 73.9% │
│ 出合い頭　　 │  6,129 │ 10.7% │
│ 出会いがしら │  5,593 │  9.8% │ - add, sK
│ 出合頭　　　 │  1,579 │  2.8% │ - [io] to [sK]
│ 出会頭　　　 │    340 │  0.6% │ - [io] to [sK]
│ 出合いがしら │    300 │  0.5% │ - add, sK
│ で会頭　　　 │    269 │  0.5% │
│ 出会いガシラ │     93 │  0.2% │
│ 出あいがしら │     80 │  0.1% │
│ 出あい頭　　 │     38 │  0.1% │
│ 出合がしら　 │     31 │  0.1% │
│ であいがしら │    498 │  0.9% │
╰─ーーーーーー─┴────────┴───────╯

@@ -3,0 +4,3 @@
+<k_ele>
+<keb>出会い頭</keb>
+</k_ele>
@@ -10 +13,2 @@
-<keb>出会い頭</keb>
+<keb>出会いがしら</keb>
+<ke_inf>&sK;</ke_inf>
@@ -14 +18 @@
-<ke_inf>&io;</ke_inf>
+<ke_inf>&sK;</ke_inf>
@@ -18 +22,5 @@
-<ke_inf>&io;</ke_inf>
+<ke_inf>&sK;</ke_inf>
+</k_ele>
+<k_ele>
+<keb>出合いがしら</keb>
+<ke_inf>&sK;</ke_inf>

Bulk update n-adv to n,adv
-*- via bulkupd.py -*-

@@ -26 +26,2 @@
-<pos>&n-adv;</pos>
+<pos>&n;</pos>
+<pos>&adv;</pos>